fix(web): properly cast Map from JSAny?.dartify() (#18855)

* fix(web): properly cast Map from JSAny?.dartify() and remove dead interop code

Fixes a runtime TypeError in firebase_performance_web's Trace.getAttributes()
where dartify() on JS objects was incorrectly cast to Map<String, String> instead
of Map<Object?, Object?>.

Also removes unused and broken dartify() casting helpers in firebase_auth_web
and firebase_remote_config_web.

* test(performance,web): add license header and @teston('browser')

* fix(remote_config,web): restore and fix RemoteConfig.defaultConfig getter

* fix(auth,web): restore User.toJson() with Map.cast and add browser unit tests

Author: kevmoo

packages/firebase_auth/firebase_auth_web/lib/src/interop/auth.dart

@@ -268,8 +268,8 @@ class User extends UserInfo<auth_interop.UserJsImpl> {
 
   /// Returns a JSON-serializable representation of this object.
   Map<String, dynamic> toJson() {
-    final result = jsObject.toJSON();
-    return (result as JSAny).dartify()! as Map<String, dynamic>;
+    return (jsObject.toJSON().dartify() as Map?)?.cast<String, dynamic>() ??
+        <String, dynamic>{};
   }
 
   @override

packages/firebase_auth/firebase_auth_web/test/user_to_json_test.dart

@@ -0,0 +1,54 @@
+// Copyright 2026 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+@TestOn('browser')
+library;
+
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+
+import 'package:firebase_auth_web/src/interop/auth.dart' as auth_interop;
+import 'package:firebase_auth_web/src/interop/auth_interop.dart'
+    as auth_js_interop;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('User.toJson', () {
+    test('converts JSObject from toJSON() to Map<String, dynamic>', () {
+      final jsJson = JSObject()
+        ..['uid'] = 'user-123'.toJS
+        ..['email'] = 'user@example.com'.toJS
+        ..['emailVerified'] = true.toJS;
+      final jsUser = JSObject()..['toJSON'] = (() => jsJson).toJS;
+
+      final user = auth_interop.User.getInstance(
+        jsUser as auth_js_interop.UserJsImpl,
+      )!;
+      final json = user.toJson();
+
+      expect(json, isA<Map<String, dynamic>>());
+      expect(
+        json,
+        equals(<String, dynamic>{
+          'uid': 'user-123',
+          'email': 'user@example.com',
+          'emailVerified': true,
+        }),
+      );
+    });
+
+    test('handles an empty JSObject from toJSON()', () {
+      final emptyJson = JSObject();
+      final jsUser = JSObject()..['toJSON'] = (() => emptyJson).toJS;
+
+      final user = auth_interop.User.getInstance(
+        jsUser as auth_js_interop.UserJsImpl,
+      )!;
+      final json = user.toJson();
+
+      expect(json, isA<Map<String, dynamic>>());
+      expect(json, isEmpty);
+    });
+  });
+}

packages/firebase_performance/firebase_performance_web/lib/src/interop/performance.dart

@@ -56,8 +56,9 @@ class Trace extends JsObjectWrapper<performance_interop.TraceJsImpl> {
   String getAttribute(String attr) => jsObject.getAttribute(attr.toJS).toDart;
 
   Map<String, String> getAttributes() {
-    return (jsObject.getAttributes().dartify()! as Map<String, String>)
-        .cast<String, String>();
+    return (jsObject.getAttributes().dartify() as Map?)
+            ?.cast<String, String>() ??
+        <String, String>{};
   }
 
   int getMetric(String metricName) =>

packages/firebase_performance/firebase_performance_web/test/trace_get_attributes_test.dart

@@ -0,0 +1,47 @@
+// Copyright 2026 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+@TestOn('browser')
+library;
+
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+
+import 'package:firebase_performance_web/src/interop/performance.dart';
+import 'package:firebase_performance_web/src/interop/performance_interop.dart'
+    as performance_interop;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('Trace', () {
+    test('getAttributes returns a Map<String, String>', () {
+      final jsAttributes = JSObject()
+        ..['a'] = 'b'.toJS
+        ..['c'] = 'd'.toJS;
+      final jsTrace = JSObject()..['getAttributes'] = (() => jsAttributes).toJS;
+
+      final trace = Trace.fromJsObject(
+        jsTrace as performance_interop.TraceJsImpl,
+      );
+      final attrs = trace.getAttributes();
+
+      expect(attrs, isA<Map<String, String>>());
+      expect(attrs, equals(<String, String>{'a': 'b', 'c': 'd'}));
+    });
+
+    test('getAttributes handles empty maps safely', () {
+      final emptyAttributes = JSObject();
+      final jsTrace = JSObject()
+        ..['getAttributes'] = (() => emptyAttributes).toJS;
+
+      final trace = Trace.fromJsObject(
+        jsTrace as performance_interop.TraceJsImpl,
+      );
+      final attrs = trace.getAttributes();
+
+      expect(attrs, isA<Map<String, String>>());
+      expect(attrs, isEmpty);
+    });
+  });
+}

packages/firebase_remote_config/firebase_remote_config_web/lib/src/interop/firebase_remote_config.dart

@@ -50,8 +50,9 @@ class RemoteConfig
   /// defaultsMap['x'] = 1;                       // remoteConfig.defaultConfig will not be updated.
   /// remoteConfig.defaultConfig['x'] = 1;        // Runtime error: attempt to modify an unmodifiable map.
   /// ```
-  Map<String, dynamic> get defaultConfig => Map.unmodifiable(
-        jsObject.defaultConfig.dartify()! as Map<String, dynamic>,
+  Map<String, dynamic> get defaultConfig => Map<String, dynamic>.unmodifiable(
+        (jsObject.defaultConfig.dartify() as Map?)?.cast<String, dynamic>() ??
+            <String, dynamic>{},
       );
 
   set defaultConfig(Map<String, dynamic> value) {

packages/firebase_remote_config/firebase_remote_config_web/test/firebase_remote_config_web_test.dart

@@ -3,8 +3,14 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 @TestOn('chrome')
+import 'dart:js_interop';
+
 import 'package:firebase_remote_config_platform_interface/firebase_remote_config_platform_interface.dart';
 import 'package:firebase_remote_config_web/firebase_remote_config_web.dart';
+import 'package:firebase_remote_config_web/src/interop/firebase_remote_config.dart'
+    as remote_config_interop_wrapper;
+import 'package:firebase_remote_config_web/src/interop/firebase_remote_config_interop.dart'
+    as remote_config_interop;
 import 'package:flutter_test/flutter_test.dart';
 import 'package:mockito/annotations.dart';
 import 'package:mockito/mockito.dart';
@@ -109,5 +115,20 @@ void main() {
       verify(remoteConfig.setDefaults(parameters));
       verifyNoMoreInteractions(remoteConfig);
     });
+
+    test('RemoteConfig.defaultConfig round-trips and returns unmodifiable Map',
+        () {
+      final jsRemoteConfig =
+          JSObject() as remote_config_interop.RemoteConfigJsImpl;
+      final wrapper = remote_config_interop_wrapper.RemoteConfig.getInstance(
+          jsRemoteConfig);
+
+      wrapper.defaultConfig = <String, dynamic>{'title': 'Hello', 'counter': 1};
+      final config = wrapper.defaultConfig;
+
+      expect(config, isA<Map<String, dynamic>>());
+      expect(config, equals(<String, dynamic>{'title': 'Hello', 'counter': 1}));
+      expect(() => config['x'] = 2, throwsUnsupportedError);
+    });
   });
 }